The invention relates to a process for the production of 1,2-dichloroethane, hereinafter referred to as EDC, which primarily serves as an intermediate product in the production of monomer vinyl chloride, hereinafter referred to as VCM, which, in turn, is used to produce polyvinyl chloride (PVC). When EDC reacts to form VCM, hydrogen chloride (HCl) is obtained. Hence, EDC is preferably produced from ethylene (C2H4) and chlorine (Cl2) in a manner such as to maintain a balance between the hydrogen chloride (HCl) produced and consumed in the various reactions, as represented by the following reaction equations:                               Cl          2                +                              C            2                    ⁢                      H            4                                      →                                C          2                ⁢                  H          4                ⁢                  Cl          2                ⁢                                  ⁢                  (                      pure            ⁢                                                  ⁢            EDC                    )                                                                                    +                      180        ⁢                                  ⁢        kJ        ⁢                  /                ⁢        Mol                                                      ⁢                  (          1          )                                                  C          2                ⁢                  H          4                ⁢                  Cl          2                ⁢                                  ⁢                  (                      cracked            ⁢                                                  ⁢            EDC                    )                            →                                C          2                ⁢                  H          3                ⁢        Cl        ⁢                                  ⁢                  (          VCM          )                            +              HCl              -                      71        ⁢                                  ⁢        kJ        ⁢                  /                ⁢        Mol                                                      ⁢                  (          2          )                                                              C            2                    ⁢                      H            4                          +                  2          ⁢                                          ⁢          HCl                +                              1            2                    ⁢                      O            2                                      →                                C          2                ⁢                  H          4                ⁢                  Cl          2                ⁢                                  ⁢                  (                      raw            ⁢                                                  ⁢            EDC                    )                            +                                H          2                ⁢        O                    +                      238        ⁢                                  ⁢        kJ        ⁢                  /                ⁢        Mol                                                      ⁢                  (          3          )                    
The process for the production of VCM with an adequate HCl balance—hereinafter referred to as “balanced VCM process”—comprises the following process steps:    direct chlorination in which one portion of the required EDC is produced from ethylene (C2H4) and chlorine (Cl2) in the presence of a homogeneous catalyst and is made available as so-called pure EDC;    oxichlorination in which the remaining portion of the required EDC is produced from ethylene (C2H4), hydrogen chloride (HCl) and oxygen (O2) and made available as so-called raw EDC;    fractionating EDC purification in which the raw EDC, together with the recycle EDC returned from the VCM fractionation step and, optionally, together with the pure EDC is freed from the secondary products formed in the oxichlorination and EDC pyrolysis steps in order to obtain a so-called feed EDC suitable for use in the EDC pyrolysis step; as an option, it is also possible to distil the pure EDC from the direct chlorination step in the heavy-ends column of the EDC distillation section;    EDC pyrolysis in which the feed EDC is thermally cracked, the mixture leaving the reactor, known as cracked gas, consisting of VCM, hydrogen chloride (HCl) and non-reacted EDC as well as secondary products;    VCM fractionation in which the desired pure VCM product is separated from the cracked gas while the other essential substances, viz. hydrogen chloride (HCl) and non-reacted EDC contained in the cracked gas, are separately recovered as valuable materials and returned as recycle HCl or recycle EDC to the balanced VCM process.
In most industrial processes, a circulating stream of EDC reaction product is used as the reaction fluid in direct chlorination. This can be accomplished in a loop-type reactor with external or internal circulation. The circulation can also be accomplished in a system with natural or forced circulation. In most cases ferric chloride is used as catalyst; in addition, sodium chloride which is able to inhibit the formation of heavy ends may be admixed as an additive. Oxygen, which is contained in minor quantities (approx. 0.5–1.5% by vol.) in the chlorine produced in the membrane electrolysis plant, or which can be admixed in the form of air to the chlorine upstream of the reactor, also has an inhibiting effect on the formation of secondary products.
The state of the art as regards direct chlorination is, for instance, described in DE 199 10 964 A1. The process according to DE 199 10 964 A1 aims at suppressing side reactions, especially the continuation of the chlorination process of EDC to 1,1,2-trichloroethane, by making most of the chlorination reaction take place in the homogeneous liquid phase. The ethylene, which is less readily soluble in EDC than chlorine, is completely dissolved in the main stream of the circulating EDC reaction fluid in a co-current bubble column. The chlorine, which is more readily soluble in EDC than ethylene, is dissolved in a supercooled EDC part-stream and the resulting solution of chlorine in EDC is fed to the circulating main stream which already contains the dissolved ethylene. The reaction (1) in the downstream largely homogenous reaction zone takes place very rapidly. It was found, however, that gaseous constituents (inerts) contained in the chlorine in admixed form as well as the oxygen from the membrane electrolysis cells were not completely dissolved in the EDC in the reaction zone but they were present in the form of fine gas bubbles causing the formation of a phase interface. This interface is seemingly able to facilitate the formation of secondary products. The secondary products obtained were primarily monochloroacetaldehyde, dichloroacetaldehyde and trichloroacetaldehyde (chloral) which cannot be separated from the EDC or only with difficulty and tend to cause problems in the downstream process steps. The formation of 1,1,2-trichloroethane was also observed. It was not known formerly that the formation of a phase interface is connected with the formation of secondary products. In the state of the art even processes are described where the formation of a phase interface is absolutely necessary. In this context it is referred to the U.S. Pat. No. 4,783,564 where some data to the required density of dissipation of energy can be found.